1. Field of the Invention
The invention relates to a detergent composition in gel form having improved clarity, the compositions being useful for the cleaning of hard surfaces.
2. The Prior Art
Detergents such as those for automatic dishwashing have traditionally been in powder or granulate form. More recently, the marketplace has seen the advent of liquid forms of automatic dishwashing products. Liquids have advantages over powders in their convenience of dispensing or dosing, their enhanced solubility, absence of lump formation or "caking" during storage, and absence of dustiness associated with the powder form.
Problems have been encountered in formulating products for automatic machine dishwashing. These problems arise because the machine dispenser cups have been designed to deliver powders. Chemists have thus been challenged in formulating a liquid product compatible with dispenser cup delivery. There are a number of factors to be considered in such formulations.
Firstly, the composition must be a uniform mixture to deliver an optimum combination of active ingredients to the wash with each dose. Thus, the liquid must possess physical stability against syneresis or physical separation of its active components during storage.
Secondly, a liquid product must be compatible with automatic dishwashing equipment presently available to the consumer. Home dishwashers are fitted with a closed cup to house detergent through several cycles preliminary to the wash cycle. Cups in these machines do not seal tightly and do not adequately retain liquids of low viscosity. Excessive leakage leads to underdosing in the wash cycle. Performance may be adversely affected. Consequently, any liquid product must possess high viscosity to be effectively retained in the cup and avoid leakage into cycles preceding that of the wash.
Conversely, there are situations where the product should have low viscosity. A low viscosity is desirable for easy dispensing of product from its bottle. Thixotropic liquids address the foregoing dilemma by maintaining high viscosity for storage but reverting to lower viscosity under influence of applied shear. Thixotropy is shear thinning behavior that is time dependent in both its decrease in viscosity under applied shear and its regain of viscosity after cessation of shearing.
The earliest approaches to these problems involved the use of clays to modify viscosity. Typical of this technology are the compositions disclosed in U.S. Pat. No. 4,116,849 (Leikhim), U.S. Pat. No. 4,431,559 (Ulrich), GB 2 116 199A (Julemont et al.) and GB 2 140 450A (Julemont et al.). Some patents such as U.S. Pat. Nos. 4,511,487 (Pruhs et al.) and 4,512,908 (Heile) have singled out hectorite as a particularly efficient thickener. There has also been reported in U.S. Pat. No. 3,558,496 (Zmoda) the advantage of combining a negatively charged clay such as hectorite with a positively charged clay such as alumina clay.
GB 2 176 495A suggests the use of polyvalent metal salts of long chain fatty acids, such as aluminum or zinc stearate, as stabilizers against phase separation in a clay laiden liquid composition. Another method of improving phase stability in thixotropic liquids is reported in GB 2 163 448A. This patent suggests inclusion of a limited amount of a water-soluble potassium salt to achieve a potassium: sodium weight ratio of about 0.04 to 0.5. Relatively large crystals are said to be inhibited from forming when potassium is present thereby resulting in greater stability against separation on ageing. U.S. Pat. No. 3,720,621 (Smeets) reports a further useful property of including some potassium salt within a sodium tripolyphosphate liquid composition. Here the presence of potassium allows the amount of sodium tripolyphosphate included within the aqueous detergent to attain a considerably higher solubility than found in the absence of potassium.
Although generally acceptable, clay structured liquids have a number of disadvantages. Montmorillonite clays, even in the presence of stabilizing agents, are sensitive to ionic strength. They lose their liquid structuring efficiency at the high electrolyte levels normally present in autodish liquid detergents. Clays tend to collapse onto themselves, or flocculate under these conditions. If this collapse occurs to any large extend during prolonged storage, the liquid will lose its physical stability, suffer syneresis and/or settling of solids. Collection of solids at the bottom of the container can lead to the formation of paste-like plugs which are difficult to dispense.
Attapulgite clay particles suspended in liquids tend to scatter light. Any large amount of these clay particles will thus impart a muddy dull color to the liquid. Furthermore, clays, being insoluble minerals, can adversely affect glass appearance. Deposition of clay onto the surface of glassware has been known to lead to spotting and filming.
Another problem of suspended solids in prior art liquids is that they are subject to recrystallization during storage periods. Through a process of Ostwald ripening, the solids can redistribute themselves in terms of number and size of crystals. These changes can cause a drastic change in rheology of the liquid over time. Poor stability and/or cup retention result.
Many polymers are known for their thickening properties. Within the machine dishwashing art, polyacrylic acid type polymers have been included as an important component but not necessarily to function as a thickener. Thus, U.S. Pat. No. 3,579,455 (Sabatelli et al.) discusses what is evidently a powdered dishwashing detergent utilizing sodium polyacrylate as an anti-spotting/streaking agent and hardness precipitator. Linear polyacrylate has also been incorporated into thixotropic liquids that have been primarily thickened with powdered clay. GB 2 163 447A (Colarusso) and GB 2 164 350A (Lai et al.) contain such clay-sodium polyacrylate systems and suggest that the polymer provides improved protection to the overglaze layer of fine china. Less filming on glassware was also noted.
Use of polymers for gel-formation in liquid detergent compositions was suggested in U.S. Pat. No. 3,060,124 (Ginn). Apparently, cross-linked vinyl polymers are primarily suitable. Hydrolyzed polyacrylonitrile cross-linked with formaldehyde was found particularly effective at stabilizing the gels against separation. U.S. Pat. No. 4,228,048 (Tesdahl) illustrates the use of polyallyl sucrose cross-linked polyacrylates, commercially available under the trademark Carbopol.RTM., as a thickener for liquid cleaning and bleaching concentrates. Japanese Laid Open Patents 59-36198 (Kao Soap) and 59-36200 (Kao Soap) further illustrate the use of polyacrylate cross-linked with compounds such as allylated pentaerythritol. These thickened formulas are used to suspend water-insoluble abrasives such as silicone dioxide and aluminum oxide.
Although the aforementioned polymer systems do provide some measure of thickening and phase stabilization, they are frequently not fully adequate at such functions, especially where there is a high level of electrolyte present. Systems are required exhibiting improved stability against phase separation at high electrolyte level and having improved rheological properties. With regard to rheology, the composition must not substantially leak from the cup of an automatic dishwasher, but at the same time be sufficiently shearing to allow flow out of its container.
Liquids including all those of the aforementioned art have another undesirable characteristic. Subsequent to pouring, the mouth of the pouring container will retain flow cut-off product droplets. Normally, these droplets will travel from the lip downward along the outside of the container. Consumers do not like the resulting mess. Some containers have been designed with special pour spouts to prevent this problem. The spouts are, however, quite expensive and not normally used for small-sized containers. It would therefore be desirable to obtain a product inherently having non-drip properties.
There has also been a search for more aesthetically pleasing product forms. Clay structurants cream the carrier liquid resulting in an opaque product. Many polymers also impart opaque properties. Clear compositions would, by contrast, be more aesthetically pleasing to the consumer.
There have been disclosed in co-pending applications Ser. No. 07/139,490, now U.S. Pat. No. 4,836,948 and 07/139,492 cleaning compositions in gel form characterized by a clarity approaching transparency. As is common for most cleaning compositions, it is desirable to include therein a surfactant. Unfortunately, many conventional surfactants when included in the formulation have been found to cause a loss in clarity. For instance, the well-known nonionic surfactant Polytergent SLF-18.RTM. severely reduces clarity.
Anionic surfactants such as the alkyldiphenyl ether disulfonates, frequently used in automatic dishwashing compositions, do not adversely affect gel clarity. On the other hand, such anionic materials tend to produce levels of foam which are in excess of what may be controllable through the use of defoamers.
Thus, it is an object of the invention to obtain a composition in gel form having a high degree of clarity.
A further object of the invention is to provide a gel cleaning composition incorporating therein a surfactant.
A still further object of the invention is to provide a gel cleaning composition pourable from a container similar to ordinary liquids but having a recoil elasticity rendering the composition dripless.
Another object of the invention is to provide an automatic dishwashing composition in the form of a clear gel with improved cleaning performance.
Also an object of the invention is to provide an automatic dishwashing composition having reduced spotting and filming with respect to glassware.